Most light weight mirrors for reflecting telescopes have been prepared from fused silica or a SiO.sub.2 --TiO.sub.2 glass, one example of which is Code No. 7971, marketed by Corning Glass Works, Corning, New York, under the trademark ULE fused silica. That glass has the approximate analysis of 92.5% SiO.sub.2 and 7.5% TiO.sub.2. The very low expansion of those materials has recommended their utility for that application since substantial variations in ambient temperatures have little effect upon the conformation and contour of the mirrors.
In the past, mirror elements have been sealed to one another utilizing high temperature fusion techniques (1500.degree.-1700.degree. C.). At such elevated temperatures, however, thermal deformation of the elements can occur and this problem is exacerbated when the individual elements are designed of less bulk so as to yield a lighter overall final structure. Stated differently, as the mass of the elements is reduced, there is less bulk therein to resist thermal deformation. This detrimental distortion can be virtually eliminated via sealing with a frit capable of flow at relatively low temperatures. Unfortunately, the commercially-available stable sealing glasses and thermally devitrifiable sealing glasses exhibit coefficients of thermal expansion which are much too high to produce sound seals with fused silica or Code No. 7971.
For example, U.S. Pat. No. 3,951,669 describes the utility of zinc beta-quartz as a refractory and low expansion filler to reduce the thermal expansion of PbO--B.sub.2 O.sub.3 and PbO--B.sub.2 O.sub.3 --SiO.sub.2 sealing glasses. As observed in that patent, the zinc beta-quartz used for that purpose demonstrated a coefficient of thermal expansion of -16.7.times.10.sup.-7 /.degree.C. In contrast, the base PbO-containing sealing glasses exhibited coefficients of thermal expansion of about 100.times.10.sup.-7 /.degree.C. and higher. The additions of zinc beta-quartz were able to lower the overall expansion to somewhat less than 50.times.10.sup.-7 /.degree.C.
As another example, U.S. Pat. No. 3,954,486 discloses sealing glasses within the PbO--B.sub.2 O.sub.3 --ZnO base field containing low expansion particulate fillers selected from the group of SiO.sub.2, beta-spodumene, petalite, Al.sub.2 O.sub.3, clay, beta-eucryptite, ZrO.sub.2, zircon, and aluminosilicate glasses. Beta-spodumene and beta-eucryptite are well-recognized as displaying coefficients of thermal expansion approximating zero. However, the base glasses to which the fillers were added had coefficients of thermal expansion of about 100.times.10.sup.-7 /.degree.C. or more. Consequently, the goal of the invention was to incorporate a sufficient amount of the filler to yield a composite material having a thermal expansion of 65.times.10.sup.-7 /.degree.C. or somewhat less.
Thermally devitrifiable sealing glasses had their origin in U.S. Pat. No. 2,889,952. Such glasses are capable of forming a conventional fused vitreous seal and thereafter undergoing a thermally induced, nucleated-type of crystallization wherein the glass separates into a vitreous phase and a crystalline phase, the latter being composed of fine-grained crystals relatively uniform in size and substantially homogeneously distributed throughout the residual vitreous matrix. The resulting layer of sealing material exhibits essentially uniform physical characteristics which are ordinarily considerably different from those of the parent glass. Most advantageously, the devitrified glass is customarily much more refractory than the precursor body. Three illustrations of such products are reported below.
U.S. Pat. No. 2,889,952 explicitly describes thermally devitrifiable sealing glasses having base compositions within the PbO--ZnO--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 system which are suitable for sealing materials having coefficients of thermal expansion of about 80-120.times.10.sup.-7 /.degree.C. No crystallographic data are provided but the specification points out that the thermal expansion of the precursor glass is relatively unimportant in sealing applications since the physical properties of the glass in its devitrified state determine the amount and nature of any stress developed in a seal.
U.S. Pat. No. 3,063,198 discusses thermally devitrifiable sealing glasses having base compositions within the PbO--B.sub.2 O.sub.3 system and containing P.sub.2 O.sub.5 and/or V.sub.2 O.sub.5 and/or Cr.sub.2 O.sub.3 as a nucleating agent. Again, no crystallographic data are provided but the coefficients of thermal expansion of the devitrified material appear to range about 90-100.times.10.sup.-7 /.degree.C.
U.S. Pat. No. 3,258,350 discloses means for modifying the coefficient of thermal expansion of devitrifiable sealing glasses by including inert filler materials therein. The patent specifically describes the incorporation of particulate zircon into base sealing glass compositions within the PbO--ZnO--B.sub.2 O.sub.3 --SiO.sub.2 field to yield a composite material demonstrating a coefficient of thermal expansion substantially below that of the base glass in the devitrified state.
As can be appreciated, none of the above-described materials would be suitable for forming sound seals with fused silica or Code No. 7971. Hence, research has been continuous to develop sealing glasses of either the stable or thermally devitrifiable type for use with those materials.
As was noted above, zinc beta-quartz is recognized as possessing one of the lowest coefficients of thermal expansion known. U.S. Pat. No. 3,681,097 discloses the production of glass-ceramic articles wherein zinc petalite solid solution or a zinc stuffed beta-quartz solid solution constitutes the predominant crystal phase. The patent describes compositions consisting essentially, expressed in weight percent on the oxide basis, of about 13-40% ZnO, 9-26% Al.sub.2 O.sub.3, 40-75% SiO.sub.2, and a nucleating agent selected from the group of 3-10% ZrO.sub.2 and 0.001-0.5% of a noble metal selected from the group of copper, gold, palladium, platinum, and silver. The essentially complete absence of alkali metal oxides is demanded and the alkaline earth metal oxides MgO, CaO, and SrO are preferably absent.
Thermally devitrifiable zinc aluminosilicate glass compositions suitable for providing a protective coating for lithium aluminosilicate glass-ceramic plates are described in U.S. Pat. No. 3,978,315. The glass-ceramic plates commonly contained beta-eucryptite, beta-eucryptite solid solution, beta-spodumene, beta-spodumene solid solution, or mixtures thereof as the principal crystal phase and demonstrated coefficients of thermal expansion (0.degree.-800.degree. C.) of about -10 to 20.times.10.sup.-7 /.degree.C. The inventive glasses consisted essentially, expressed in weight percent on the oxide basis, of about 12-25% ZnO, 0-3% MgO, 0-3% CoO, 15-25% ZnO+MgO+CoO, 15-28% Al.sub.2 O.sub.3, 50-65% SiO.sub.2, and at least about 5% total of oxides not exceeding the indicated proportions selected from the group of up to 1% K.sub.2 O, up to 5% Cs.sub.2 O, and up to 4% BaO.
The method for coating the glass-ceramic plate contemplated forming a slurry or paste of powdered glass in a suitable oil vehicle and then applying the slurry or paste onto the plate by brushing, spraying, silk screening, doctor blading, or other conventional technique. The resulting coating is thereafter fired to remove the vehicle, to sinter and bond the glass to the plate, and to crystallize the glass in situ to secure the desired crystallized layer. Sintering of the glass can take place at about 925.degree.-950.degree. C. and crystallization thereof at temperatures within the range of about 825.degree.-950.degree. C.
In general, the formation of a glass seal utilizing a devitrifiable sealing glass involves five steps:
(a) the sealing glass batch is melted;
(b) the melt is cooled to a glass and the glass comminuted to a very fine powder;
(c) the powdered glass is mixed with a conventional binder and vehicle;
(d) the resultant suspension or slurry is applied onto the surfaces to be sealed together; and
(e) the surfaces to be sealed are brought together and fired to burn out the binder and vehicle and then to form a thermally devitrified seal therebetween.
Where a composition in the vicinity of the stoichiometry of zinc beta-quartz is melted and powdered glass prepared from the melt, the resultant powder crystallizes too rapidly to have utility as a sealing material. Thus, there is not enough flow taking place before devitrification to permit satisfactory covering and wetting of the surfaces to derive a sound seal therebetween.